The use of spectrometry to measure tissue oxygen saturation levels in a non-invasive manner is known in a general sense. Indeed, efforts to exploit the oxygenation dependent light absorption properties of hemoglobin to determine the oxygen saturation of hemoglobin in vivo have been made for several decades. In the 1980's so-called “pulse oximeters” became commercially available, and have continued to develop. An important advantage of such instruments is their ability to provide continuous, safe, and effective monitoring of arterial oxygenation non-invasively at a patient's bedside.
Pulse oximeters operate on the known principle that oxygenated and deoxygenated hemoglobin show different absorption spectra. Deoxygenated hemoglobin absorbs more light in the red band (typically 650–750 nm), while oxygenated hemoglobin absorbs more light in the infrared band (typically 850–1000 nm). Pulse oximeters generally use one wavelength in the near infrared band and one in the red band to measure the oxygen saturation of arterial blood. Traditionally, pulse oximetery has faced problems associated with determining the scatter that occurs in tissue. Without such a determination, accurate measures of absorption are not possible. Typically this problem has been addressed by “calibrating” devices on a population of healthy subjects to empirically determine levels of scatter.
Recent advances in pulse oximetery have been made. For example, the introduction of time-resolved optical spectroscopy in conjunction with diffusion theory has lead to quantitative tissue spectroscopy, as described in U.S. Pat. No. 5,497,769 to Gratton et al. (“the '769 patent”), which is incorporated by reference herein. The '769 patent generally discloses an apparatus useful for measuring the hemoglobin saturation in tissue that is particularly sensitive to blood in the capillaries where oxygen is exchanged with the tissue. Thus the '769 patent generally teaches an apparatus useful for measuring tissue oxygen saturation to give an indication of tissue oxygen consumption, but does not measure time-varying hemoglobin compartment saturation.
More recently, U.S. Pat. No. 6,216,021 to Franceschini et al. discloses a spectrometry-based method for the real time, non-invasive, simultaneous measurement of tissue hemoglobin saturation and time varying arterial hemoglobin saturation. The '769 patent provides a solution to the tissue scatter problem. Generally, the light signal is measured at two locations to determine tissue optical properties. The optical properties determine the amount of scatter that occurs as light travels through tissue. To determine time varying hemoglobin compartment saturation, an amplitude of absorption oscillations at the frequency of arterial pulsation is quantitatively calculated at multiple wavelengths from the oscillations of the optical signal collected by the spectrometer using the determined tissue optical properties, with the result that an absolute value of the arterial hemoglobin saturation may then be determined. Determination and use of the tissue optical properties allows the method of the '021 patent to be used without prior calibration of a spectrometer on a population of healthy subjects.
The teachings of the prior art, however, have generally been ineffective in regards to measurement of venous compartment saturation levels. Experimental approaches have been proposed for such measurements. For instance, the use of spectrometry in combination with physical manipulation of a patient has been proposed. Physical manipulations associated with these methods include use of a venous occlusion on a limb, tilting a patient's head downward, use of a partial jugular vein occlusion, use of mechanical ventilation, and the like. Generally, these methods contemplate optically measuring the venous saturation by measuring the increase in oxygenated hemoglobin concentration and in the total hemoglobin concentration induced by the local increase in the venous blood volume. All of these proposed methods, however, have significant disadvantages associated with them. Required physical manipulation of the patient, for instance, can be cumbersome, painful, and at times impractical. Additionally, these methods may be limited to use on the limbs (e.g., venous occlusion methods).
Additional unresolved problems in the art relate to verifying the accuracy of venous compartment measurements. That is, the proposed methods of the prior art generally have a considerable level of uncertainty associated with them. This uncertainty results from the many difficulties faced in isolating the venous compartment contribution to optical tissue absorption measurements.
Unresolved needs in the art therefore exist.